This invention relates to a method for manufacturing a semiconductor device having an oxide film on the surface of a semiconductor body, and to the semiconductor device manufactured thereby.
In general, silicon crystal bodies having major surfaces lying parallel to a {111} crystal plane have been widely used in manufacturing semiconductor devices in which the major surfaces of the silicon bodies are covered with oxide films of insulator material which is moisture-resistant and is chemically stable such as, for example, of silicon dioxide SiO.sub.2. A planar transistor in which diffused regions are formed in the major surface of the body lying parallel to a {111} crystal plane covered with a passivating film is an example of such semiconductor devices.
In the case where the above-mentioned silicon dioxide film is formed on the surface of a semiconductor substrate, a donor-type surface charge appears in the surface of the semiconductor substrate immediately below the silicon dioxide film, irrespective of the conductivity type of the substrate. This phenomenon is sometimes called as the channel effect. Accordingly, it has been proposed to produce field effect transistors by utilizing the channel effect.
A large amount of the donor-type induced surface charge, however, usually gives disadvantages to semiconductor devices, such as planar transistors and MOS field effect transistors. For example, the N-type channel or inversion layer appeared in a surface of a P-type silicon substrate due to the induced surface charge gives rise to adverse results such as increase in the collecor cut-off current I.sub.co of the planar transistor. Further, due to the large amount of the induced surface charge, an MOS field effect transistor having a high mutual conductance g.sub.m can hardly be obtained. Furthermore, the large amount of the donor-type surface charge within the channel layer means that in a field effect transistor the drain current at the time of zero gate voltage has a certain large value.
For reducing the amount of the induced surface charge, a heat treatment combined with application of voltage accross the oxide film can be applied to such semiconductor devices. However, even by this heat treatment, there has been a limit to the amount of controllable surface charge, that is, a limit below which the amount of the donor-type surface charge cannot be reduced.